Ornithine Aminotransferase Deficiency in Gyrate Atrophy: Gyrate atrophy (GA) is an autosomal recessive degenerative disease of the retina and choroid of the eye which leads to blindness. We have isolated a gene probe for the human ornithine aminotransferase (OAT), a mitochondrial enzyme which is deficient in GA patients. The gene probe is a Lambagt11 cDNA clone which was obtained from our human cDNA library through a Western screening method using the anti-human OAT antibodies. The OAT cDNA is 2073 base pairs long and contains the complete coding sequence of the protein. The cDNA-derived OAT sequence is a precursor containing a leader sequence like other mitochondrial enzymes, matches the sequences of seven purified tryptic peptides of pure OAT, and shows homology with another mitochondrial enzyme, aspartate aminotransferase. Examination of the genomic organization of OAT using the cDNA as a probe revealed a gene family consisting of approximately four copies of OAT or OAT-like gene sequences. The OAT gene sequences were mapped to multiple chromosomal loci, confirming the presence of a gene family. Examination of the OAT genes of GA patients has revealed restriction fragment length polymorphisms but no grossly obvious abnormalitie, including deletions. Characterizations of multiple gene clones of OAT and the status of OAT mRNA synthesis in GA patients are in progress. Hereditary Retinoblastoma: We are investigating the molecular basis of malignant transformation in hereditary retinoblastoma using cell culture and molecular genetic techniques. To determine if retinoblastoma has a dominant or recessive malignant phenotype, retinoblastoma cells (Y79) were fused with non-malignant cells (NIH3T3), and the growth characteristics of the hybrid cells were studied. The hybrid cells, containing both the neomycin and GPT markers from the parents, are anchorage-dependent, have a fibroblastic morphology and do not grow in soft agar like the non-malignant parent. The results indicate the maligant phenotype of retinoblastoma to be recessive.